5 Most Common PoE Questions

Author: Brian Rhodes, Published on Feb 02, 2017

In our IPVM IP Networking course, Power Over Ethernet (PoE) is a core component that students show significant interest in.

The top ones are:

  1. Can I accidentally double PoE wattage by using midspans & switches together?
  2. Does each port produce max rated wattage?
  3. Can a cable plugged into a port, but not a camera, electrocute me or be a safety hazard?
  4. How far can PoE travel on a cable?
  5. Will cameras using power supplies be damaged by also plugging them into PoE ports?

In the sections below, we answer each question.

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Comments (18)

"Can a cable plugged into a port, but not a camera, electrocute me or be a safety hazard?" Answer: NO...

That's not necessarily true. There are plenty of switches that have passive POE in which case there is no negotiation happening first. One popular example are the Ubiquiti Tough Switches, if the POE is activated on a port, voltage is always present and you can feel it, ask me how I know ;)

Products that use passive PoE are uncommon because of this.

I've always wondered what the reasons were for using passive PoE. Are there any advantages to it? I seem to see it more wireless products like ubiquiti.

I think it must be because of some legacy interpretations of pre- or early IEEE PoE 802.3af. There was ambiguity in the negotiation process that some manufacturers implemented differently.

Even when wattage is produced on the line, it is not generally full strength. There is still a 'negotiation' process that happens when the device properly identifies which Class it needs.

For 'proprietary' (ie: Ubiquiti only) PoE, all devices are designed to use the same PSE design . So the Ubiquiti version you mention may not adhere to IEEE 802.3 standards regardless. Non-standard PoE is sometimes used.

As time passes, PoE for video (especially with 60W or 100W versions come online), passive devices become rarer.

Even when wattage is produced on the line, it is not generally full strength. There is still a 'negotiation' process that happens when the device properly identifies which Class it needs.

IMHO, although the number of devices which actually negotiate a power class are increasing, I believe most are still class 0, and draw based on their load once POE is supplied.

sometimes it's simply because there are cheaper passive POE products out there and some people don't know there is a difference between IEEE POE standards and passive POE.

My last comment was to make sure people know that you can get zapped if you're going into an existing system without knowing what's in place and cutting into the wire lets say to re-terminate a cable. In some cases, you can get electrocuted by POE but this shouldn't be possible if it's following the IEEE POE standards.

...In some cases, you can get electrocuted by POE but this shouldn't be possible if it's following the IEEE POE standards.

Unless you just happen to present a 25k ohm load, which would trigger the flow ;)

Either way, IMHO, you are not getting electrocuted, in the strict sense of the word at least.

This is one reason why POE work doesn't normally require an electricians license.

POTS lines can give you a little buzz as well, if you are lucky enough to be a path right when a call comes in.

Agreed, I should have said shocked. I want to edit my response but it won't let me. I will say I've been shocked by POE and it caught me off guard while standing on a tall ladder.

Had one in my mouth once when a call came in. Cured me of that habit!

Because it's cheap. Why bother with all that troublesome negotiation stuff when you can just put 48V across pairs 1 and 4, and let'er rip?

Been there done that - dont plug a 24V UBNT PoE Device into a toughswitch configured for 48V - they kind of stop working

My Smart TV's (former) network port feels your pain.

Passive Aggressive.

The device must be powered to request the power? How does the negotiations work?

In many cases Layer 2 LLDP is used, where the PSE sends a brief 'check' signal of something like 0.05W to the device so it can respond as needing PoE.

This power signal is dissipated by the jack/NIC as harmless if not acknowledged.

Thanks - that would make sense. So in effect it delivers a tiny amount of power by default to boot the Poe controller. It then states its requirements and the switch says ok here's xx watts.

cheers Brian 👍🏻

So in effect it delivers a tiny amount of power by default to boot the Poe controller. It then states its requirements and the switch says ok here's xx watts.

No, not exactly.

The switch delivers a ~2v pulse to test the resistance of the camera.

If the resistance is between 19k and 26k ohm, it either supplies

  1. 48-57v and up to 15w to the camera immediately
  2. or performs additional checks at 20v to determine its power class (if power class >0 is supported by the device), and then supplies power in that range.

In either case, the camera doesn't start booting until this is complete. Also as I mentioned earlier, most Poe af cameras that I have dealt with are just class 0, so they only do step 1.

There is a higher level lldp negotiation that can occur also, but I haven't encountered it in practice.

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